Thermostat problems

I had a Honeywell digital thermostat in a good interior wall location. It
seemed to behave well except when it was really cold outside. When that
happened, my setting of 68 would invariably go to 72, and sometime more.
Thinking that the thermostat was defective, I bought a new one and it still
continues. Can anyone advise why the thermostat won't cut out the furnace
when the setting temperature has been reached ... only when it's extremely
cold outside?

Hi,
On some like my Vision Pro 8000, I can set the temperature band tight or
little loose from service option.(Like +/- 2 degs. or +/- 1 degs. from
target temp.) You can look into this. Here right now it is -20C outside
but 'stat works to 20.5C bang on all the time. No over/under shoots.

If this is a new, off the shelf thermostat and it's doing this with
the default settings, as did the old one, something is very strange.
ARe you saying the thermostat is set to 68, but the furnace continues
to run and the thermostat display continues to go on up to 72, which
probably takes another 30 mins+ of furnace run time? I could see how
a defective thermostat might do this, or one where you screwed with
the settings, but not one right out of the box.

What model thermostat, and what type of heating system? Do you have an
outdoor temp sensor, a remote indoor temp sensor, and/or a humidifier? Have
you checked with Honeywell? http://yourhome.honeywell.com.

The replacement digital thermostat was not a Honeywell. It's a Sears-bought
Ritetemp. Being as it's exhibiting the same problems as the previous
Honeywell, I have no doubt that the thermostat is not the problem. Take a
look at my earlier comment about the "cut-in" thing, attached to the gas
furnace. I feel pretty sure that this problem lies with this thing.

Just a thought, but are you sure nobody else in the house may have
bumped the temperature up? Ours is set to 68, but on a cold weekend
I'll bump it up to 70 using the temporary setting. The program will
still show a setting of 68 because I did not re-program it, but used
the temporary setting to increase the temperature.
-Felder

No. Just the wife and I and she does not touch it.
I have just taken a better look at the furnace, and there a Honeywell device
attached to it. The device is about the size of a pack of cards, and on the
front, it has the words "Cut In", and a screw here you can adjust a setting
which is in PSI, starting at 0.5, running to 9. What is this thing and
could it be the cause of my problem?

Okay. You have a boiler, not a furnace. You may think it pedantic, but
it's an important distinction.
You say your heat goes up to 72+ when your thermostat is set to 68 in
colder weather. Which of the following is happening?
1) The thermostat cuts out at 68, same as always, but the system
overshoots to 72+, due to residual heat in the system
OR
2) The thermostat doesn't cut out until 72+.
If it's #1 (which is my guess), then it appears that for some reason
(maybe even by design) your boiler temperature is getting hotter than
normal in cooler weather. Since the temperature of your boiler water is
controlled by the pressurestat, there may a problem with the
pressurestat. I am definitely *not* qualified to offer repair advice on
steam systems, so if scenario #1 is the case, then my suggestion is to
get a good HVAC tech to check out the system. You can help pay for that
by returning your new thermostat to Sears, as the old thermostat wasn't
broken.
If it's #2, then make sure your thermostat has it's cph, or swing, or
differential set for steam. That's almost never the default - they
usually come pre-configured for normal efficiency forced hot-air
systems.

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